The historically evolved three layers of linked data: The internet (III), representing a network of computers, the web (WWW), representing hyperlinked documents, and the social-semantic web (GGG), representing interlinked data of objects. The social web is an application of the semantic web.

Illustration of a small social network with three cliques connected via bridges. There are strong ties between the individuals Alice and Bob, and Alice and Carol. Based on the definition by Granovetter (1973), there is at least a weak tie between Bob and Carol.

The architecture of centralized and decentralized OSNs. While in both scenarios two SNSs exist, the underlying OSNs in the centralized scenario are separated, while there is an interconnected OSN in the decentralized scenario.

The topologies of centralized, decentralized, distributed, and decentralized-distributed-hybrid SNSs (cf. Baran 1964: 2). Circles are representations of participants in the social network, boxes are SNS providers. In the distributed and hybrid scenarios, white boxes indicate no distinction between users and providers.

A user in the social web needs mechanisms for authentication and authorization (Section 4.1), tools for presenting profile information and social relations (Section 4.2), and protocols for real-time publishing and interactions (Section 4.3).

Four basic steps in a (simplified) OpenID authentification: 1) the user types her OpenID into the login form of a relying party site, 2) the relying party site redirects the user to the associated OpenID provider, 3) the user authentifies herself to the provider by inserting, for example, a password, then 4) after authentifying successfully, the OpenID provider tells the relying party that the user is rightfully associated to the given OpenID.